It seems unnecessary to translate Janin's French, but in case someone can't understand it, here's a rough English equivalent:

One day someone asked Geoffroy why he didn't write comedies himself, since he was such a good judge of them. He answered, "When you give a whipping to others, you shouldn't expose your own backside."

Karl Narveson writes:

I think "quand on donne" refers to a habit, not a single occasion, and therefore I question whether the "whipping" in your rough translation should be singular. "When you're in the business of administering whippings" might be more like it.